Silane (or monosilane or silicon tetrahydride: SiH.sub.4) is used mainly as a silicon vector in providing deposits from the vapor phase. For the production of semiconductors, particularly in the VLSI (Very Large Scale Integration) techniques, deposits of polycrystalline silicon, silica, silicon nitride are made by using silane as the silicon vector.
Thin-layer deposits of polycrystalline silicon, obtained from silane, make it possible to produce solar batteries having an energy output greater than 6%. It is possible to obtain coatings, which are resistant to corrosion by acids, on metals by cracking of silane. Finally, silane can be added to multiple bonds of unsturated hydrocarbons to yield organosilanes.
Numerous methods of synthesis of silane have been proposed but only three seem to have had an industrial development. The molten salt method consists of reducing a chlorosilane by lithium hydride, at a temperature on the order of 450.degree. C., in a molten bath of lithium chloride-potassium chloride. This technique offers the advantage of directly producing silane of good purity; on the other hand, it has the disadvantage of high cost and of handling molten salts that is generally not easy. A variant of this technique was envisaged by Sundemeyer, Glemser Angew. Chem. 70, p. 625 (1958). It consists in electrolyzing lithium chloride in situ to produce lithium which is transformed into lithium hydride by introduction of hydrogen into the reactor. This process has not been worked industrially, mainly because of technological difficulties.
According to the method utilizing reduction of silicon chloride by lithium aluminum hydride, the reaction is performed in the vicinity of ambient temperature in a heavy solvent such as diglyme or tetraglyme. This production process is no longer used because of its much too high cost and because of the degradation of the solvent, which strongly pollutes the silane by hydrocarbons, and which therefore requires a thorough purification of the product.
In the method of hydrolysis of silicon alloys, various binary alloys have been considered such as CaSi, CaSi.sub.2, MgSi and Mg.sub.2 Si. According to E. Wiberg, Hydrides, Elsevier N.Y. 1971, p 473, only magnesium silicide of these alloys leads to a yield of some interest. However, the attack of this compound gives silane transformation rates that can vary according to operating conditions. It appears that the yield is strongly linked to the size of the magnesium silicide grains, the rate of introduction of the powder of the compound, the reaction temperature and the direction of putting the reagents into contact.
According to a variant of this process, a silane yield on the order of 80% in relation to the silicon is obtained by performing the attack of the magnesium silicide by ammonium chloride in a liquid ammonia medium.
The principle of utilizing hydrolysis of magnesium silicide offers the advantage of depending on simple chemistry and producing silane whose main impurity is water which is easy to eliminate. On the other hand, the transposition of magnesium silicide hydrolysis to an industrial scale has two major economic and technical drawbacks. This initial product, whose preparation requires special equipment now applying only to the production of silane, is an expensive product. On the other hand, since obtaining good yields depends on the performance of the reaction in the presence of liquid ammonia, it is necessary to work under a pressure on the order of 6 atmospheres, then perform a very effective silane/ammonia separation.
In 1956, Chretien, Freundlich and Deschanvres, during their work on ternary alloys Ca.sub.3 Al.sub.6 Si.sub.2 and Ca.sub.2 Al.sub.4 Si.sub.3, noted that these acid-sensitive compounds produced, when attacked by dilute hydrochloric acid, a release of gas (silane) spontaneously flammable in the air (CR Acad, Sciences, Feb. 6, 1956, pp 784-5). The report of these laboratory experiments did not give the conditions necessary for embodying a profitable process, and they did not lead to any industrial development.